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BLADEOS 6.5.2 Application Guide
276 Chapter 20: OSPF BMD00220, October 2010
Neighbors and Adjacencies
In areas with two or more routing devices, neighbors and adjacencies are formed.
Neighbors are routing devices that maintain information about each others’ health. To establish
neighbor relationships, routing devices periodically send hello packets on each of their interfaces.
All routing devices that share a common network segment, appear in the same area, and have the
same health parameters (hello and dead intervals) and authentication parameters respond to
each other’s hello packets and become neighbors. Neighbors continue to send periodic hello packets
to advertise their health to neighbors. In turn, they listen to hello packets to determine the health of
their neighbors and to establish contact with new neighbors.
The hello process is used for electing one of the neighbors as the area’s Designated Router (DR) and
one as the area’s Backup Designated Router (BDR). The DR is adjacent to all other neighbors and
acts as the central contact for database exchanges. Each neighbor sends its database information to
the DR, which relays the information to the other neighbors.
The BDR is adjacent to all other neighbors (including the DR). Each neighbor sends its database
information to the BDR just as with the DR, but the BDR merely stores this data and does not
distribute it. If the DR fails, the BDR will take over the task of distributing database information to
the other neighbors.
The Link-State Database
OSPF is a link-state routing protocol. A link represents an interface (or routable path) from the
routing device. By establishing an adjacency with the DR, each routing device in an OSPF area
maintains an identical Link-State Database (LSDB) describing the network topology for its area.
Each routing device transmits a Link-State Advertisement (LSA) on each of its active interfaces.
LSAs are entered into the LSDB of each routing device. OSPF uses flooding to distribute LSAs
between routing devices. Interfaces may also be passive. Passive interfaces send LSAs to active
interfaces, but do not receive LSAs, hello packets, or any other OSPF protocol information from
active interfaces. Passive interfaces behave as stub networks, allowing OSPF routing devices to be
aware of devices that do otherwise participate in OSPF (either because they do not support it, or
because the administrator chooses to restrict OSPF traffic exchange or transit).
When LSAs result in changes to the routing device’s LSDB, the routing device forwards the
changes to the adjacent neighbors (the DR and BDR) for distribution to the other neighbors.
OSPF routing updates occur only when changes occur, instead of periodically. For each new route,
if an adjacency is interested in that route (for example, if configured to receive static routes and the
new route is indeed static), an update message containing the new route is sent to the adjacency. For
each route removed from the route table, if the route has already been sent to an adjacency, an
update message containing the route to withdraw is sent.